Photostable liquid-crystalline medium

ABSTRACT

The invention relates to a photostable liquid-crystalline medium comprising
         at least one compound of the formula I       

     
       
         
         
             
             
         
       
         
         
           
              and 
             at least one compound of the formula II 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
              and to the use thereof for electro-optical purposes, and to displays containing this medium.

This application is a divisional of U.S patent application Ser. No.10/531,376, filed Apr. 15, 2005, now U.S. Pat. No. 7,211,302, which isincorporated by reference herein.

The present invention relates to a photostable liquid-crystallinemedium, to the use thereof for electro-optical purposes, and to displayscontaining this medium.

Liquid-crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and lower vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   -   1. MOS (metal oxide semiconductor) or other diodes on a silicon        wafer as substrate.    -   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joins.

In the case of the more promising type 2, which is preferred, theelectrooptical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out world-wide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display with integratednon-linear elements, i.e., besides the active matrix, also displays withpassive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKOGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p.141ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design ofThin Film Transistors for Matrix Addressing of Television Liquid CrystalDisplays, p.145 ff, Paris]. With decreasing resistance, the contrast ofan MLC display deteriorates, and the problem of after-image eliminationmay occur. Since the specific resistance of the liquid-crystal mixturegenerally drops over the life of an MLC display owing to interactionwith the interior surfaces of the display, a high (initial) resistanceis very important in order to obtain acceptable service lives. Inparticular in the case of low-volt mixtures, it was hitherto impossibleto achieve very high specific resistance values. It is furthermoreimportant that the specific resistance exhibits the smallest possibleincrease with increasing temperature and the lowest possible sensitivityon heating and/or UV exposure. The low-temperature properties of themixtures from the prior art are also particularly disadvantageous. It isdemanded that no crystallisation and/or smectic phases occur, even atlow temperatures, and the temperature dependence of the viscosity is aslow as possible. The MLC displays from the prior art do not meet today'srequirements.

There thus continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times even at low temperatures and low thresholdvoltage which do not have these disadvantages, or only do so to areduced extent.

Numerous liquid-crystal mixtures used in MLC displays comprise compoundshaving limited photostability. Although these compounds are generallystable to natural light—even over an extended period—and can usuallyalso be exposed to UV irradiation for some time without this resultingin decomposition of individual mixture constituents, exposure of themixtures to, in particular, intense UV radiation for an extended periodcan result, however, in undesired photochemical processes whichpartially decompose the compounds of limited photostability and can thusmodify the liquid-crystal mixtures in their composition and in theirproperties in a sensitive manner or even render them unusable. Thisproblem has intensified recently through the fact that the so-called“one-drop filling method” [H. Kamiya, K. Tajima, K. Toriumi, K. Terada,H. Inoue, T. Yokoue, N. Shimizu, T. Kobayashi, S. Odahara, G. Hougham,C. Cai, J. H. Glownia, R. J. von Gutfeld, R. John, S.-C. Alan Lien, SID01 Digest (2001), 1354-1357], during the use of which the display cellsfilled with a liquid-crystal mixture are irradiated with UV light for anextended period in order to effect polymerisation of the monomers usedas sealing agent (for example acrylates or epoxides) in order to sealthe cells, is now being employed in the manufacture of liquid-crystaldisplays.

Compounds having a —(CH₂)₂—, —COO—, —CH═CH—, —CF═CH—, —CH═CF—, —CF═CF—,—C≡C— or —CF₂O— link between two ring constituents of the molecule,which are frequently apparently mesogenic, are comparatively sensitiveto UV radiation. It is therefore also desirable to improve thephotostability of liquid-crystal mixtures without impairing theirabove-mentioned properties which are necessary, in particular, for usein MLC displays.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   a long shelf life, even at extremely low temperatures    -   the ability to switch at extremely low temperatures (outdoor        use, automobile, avionics)    -   increased resistance to UV radiation (longer life).

The media available from the prior art do not allow these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired which enablegreater multiplexability and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther widening of the available parameter latitude (clearing point,smectic-nematic transition or melting point, viscosity, dielectricparameters, elastic parameters) is urgently desired.

The invention is based on the object of providing media, in particularfor MLC, TN or STN displays of this type, which do not have theabove-mentioned disadvantages, or only do so to a reduced extent, andpreferably at the same time have very low threshold voltages and at thesame time high values for the voltage holding ratio (VHR) and haveimproved stability to UV radiation.

It has now been found that this object can be achieved if mediaaccording to the invention are used in displays.

The invention thus relates to a liquid-crystalline medium comprising

-   -   at least one compound of the formula I

and

-   -   at least one compound of the formula II

in which

L¹, L², L³ and L⁴ are each, independently of one another, H or F;

R¹¹ is H, a halogenated or unsubstituted alkyl radical having from 1 to15 carbon atoms, where, in addition, one or more CH₂ groups in theseradicals may each be replaced, independently of one another, by —C≡C—,—CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are not linkeddirectly to one another;

R²¹ and R²² are each, independently of one another, H, Cl, F, CN, SF₅,SCN, NCS, a halogenated or unsubstituted alkyl radical having from 1 to15 carbon atoms, where, in addition, one or more CH₂ groups in theseradicals each may be replaced, independently of one another, by —C≡C—,—CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are not linkeddirectly to one another;

Y¹¹ is F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, ahalogenated alkenyl radical, a halogenated alkoxy radical or ahalogenated alkenyloxy radical, each having up to 6 carbon atoms;

Z¹¹ is a single bond, —CH₂—CH₂—, —CH═CH—, —CH═CF—, —CF═CH—, —CF═CF—,—C≡C—, —COO—, —OCO—, —CF₂O— or —OCF₂—;

a and f, independently of one another, are 0 or 1;

b, c, d and e are each, independently of one another, 0, 1 or 2;

Particular preference is given to a liquid-crystalline medium accordingto the invention comprising

-   -   at least one compound of the formula IA

and

at least one compound of the formula II

in which

L² is H or F;

R¹¹ is H, a halogenated or unsubstituted alkyl radical having from 1 to15 carbon atoms, where, in addition, one or more CH₂ groups in theseradicals may each be replaced, independently of one another, by —C≡C—,—CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are not linkeddirectly to one another;

R²¹ and R²² are each, independently of one another, H, Cl, F, CN, SF₅,SCN, NCS, a halogenated or unsubstituted alkyl radical having from 1 to15 carbon atoms, where, in addition, one or more CH₂ groups in theseradicals may each be replaced, independently of one another, by —C≡C—,—CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are not linkeddirectly to one another;

Y¹¹ is F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, ahalogenated alkenyl radical, a halogenated alkoxy radical or ahalogenated alkenyloxy radical, each having up to 6 carbon atoms;

Z¹¹ is a single bond, —COO— or —CF₂O—;

f is 0 or 1;

b, c, d and e are each, independently of one another, 0, 1 or 2;

(If Z¹¹ in the formula I or IA is a single bond and f in the formula IIis zero, these compounds of the formula I or IA on the one hand and ofthe formula II on the other hand which are present in the mediumaccording to the invention are selected in such a way that they are notidentical.)

It is preferred for the liquid-crystalline medium according to theinvention to form the basis of a mixture of polar compounds of positivedielectric anisotropy or to be a constituent of such a mixture. Thecompounds of the formulae I and II have a broad range of applications.Depending on the choice of substituents, these compounds can either beused as base materials of which liquid-crystalline media arepredominantly composed; however, it is also possible to add compounds ofthe formulae I and II to liquid-crystalline base materials from otherclasses of compound in order, for example, to modify the dielectricand/or optical anisotropy of a dielectric of this type and/or tooptimise its threshold voltage and/or its viscosity and in particular toimprove its photostability.

The liquid-crystalline medium of the present invention comprising thecompounds of the formulae I and II proves to be particularlyadvantageous with respect to the UV stability of liquid-crystalmixtures.

In the pure state, the compounds of the formulae I and II are colourlessand form liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use.

If R¹¹, R²¹ and/or R²² is an alkyl radical and/or an alkoxy radical(i.e. an alkyl radical in which (at least) the —CH₂— group via whichR¹¹, R²¹ or R²² respectively is bonded to the ring in question has beenreplaced by O), this may be straight-chain or branched. It is preferablystraight-chain, has 1, 2, 3, 4, 5, 6 or 7 carbon atoms and accordinglyis preferably methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl,methoxy, ethoxy, propoxy, butoxy, pentoxy, hexyloxy or heptyloxy,furthermore octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl,pentadecyl, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy,tridecyloxy or tetradecyloxy.

Oxaalkyl—i.e. an alkyl chain in which at least one CH₂ group has beenreplaced by O, but where the oxygen atom is not bonded directly to thering substituted by R¹¹, R²¹ or R²²—is preferably straight-chain2-oxapropyl (=methoxymethyl), ethoxymethyl or 3-oxabutyl(=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-,3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-,-4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or9-oxadecyl.

If R¹¹, R²¹ and/or R²² is an alkyl radical in which one CH₂ group hasbeen replaced by —CH═CH—, this may be straight-chain or branched. Aradical of this type is also known as an alkenyl radical. It ispreferably straight-chain and has from 2 to 10 carbon atoms.Accordingly, it is particularly preferably vinyl, prop-1- or -2-enyl,but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or 4-enyl, hex-1-, -2-, -3-,-4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-,-3-, 4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, 4-, -5-, -6-, -7- or-8-enyl, or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl. If thealkenyl group can have the E or Z configuration, the E configuration(trans configuration) is generally preferred.

If R¹¹, R²¹ and/or R²² is an alkyl radical in which one CH₂ group hasbeen replaced by —O— and one has been replaced by —CO—, these arepreferably adjacent. These thus contain an acyloxy group —CO—O— or anoxycarbonyl group —O—CO. These are preferably straight-chain and havefrom 2 to 6 carbon atoms. Accordingly, they are in particular acetoxy,propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl,propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl,2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R¹¹, R²¹ and/or R²² is an alkyl radical in which one CH₂ group hasbeen replaced by unsubstituted or substituted —CH═CH— and an adjacentCH₂ group has been replaced by CO, CO—O or O—CO, this may bestraight-chain or branched. It is preferably straight-chain and has from4 to 12 carbon atoms. Accordingly, it is in particularacryloyloxymethyl, 2-acryloyloxyethyl, 3-acryloyloxypropyl,4-acryloyloxybutyl, 5-acryloyloxypentyl, 6-acryloyloxyhexyl,7-acryloyloxyheptyl, 8-acryloyloxyoctyl, 9-acryloyloxynonyl,10-acryloyloxydecyl, methacryoyloxymethyl, 2-methacryloyloxyethyl,3-methacryloyloxypropyl, 4-methacryloyloxybutyl,5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

If R¹¹, R²¹ and/or R²² is an alkyl, alkoxy or alkenyl radical which isat least monosubstituted by halogen, this radical is preferablystraight-chain, and halogen is preferably F or Cl. In the case ofpolysubstitution, halogen is preferably F. The resultant radicals alsoinclude perfluorinated radicals. In the case of monosubstitution, thefluorine or chlorine substituent may be in any desired position, but ispreferably in the ω-position. Particular preference is given to —CF₃,—CHF₂, —CH₂F, —CH₂CF₃, —CHFCHF₂, —CF₂CF₃, —OCF₃, —OCHF₂, —OCH₂F and—CF═CF₂.

Compounds containing branched wing groups R¹¹, R²¹ and/or R²² mayoccasionally be of importance owing to better solubility in the usualliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

Branched groups of this type preferably contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

If R¹¹, R²¹ and/or R²² is an alkyl radical in which two or more CH₂groups have been replaced by —O— and/or —CO—O—, this may bestraight-chain or branched. It is preferably branched and has from 3 to12 carbon atoms. Accordingly, it is in particular biscarboxymethyl,2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl,5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl,8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl,bis(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl,3,3-bis(methoxycarbonyl)propyl, 4,4-bis(methoxycarbonyl)butyl,5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl,7,7-bis(methoxycarbonyl)heptyl, 8,8-bis(methoxycarbonyl)octyl,bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl,3,3-bis(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl or5,5-bis(ethoxycarbonyl)pentyl.

R²¹ and R²² may also, independently of one another, be F, Cl, CN, SF₅,SCN or NCS, in particular F or Cl.

If Y¹¹ is a halogenated alkyl, alkenyl, alkoxy or alkenyloxy radical,this radical may be straight-chain or branched. The radical ispreferably straight-chain, and is substituted by F or Cl. In the case ofpolysubstitution, halogen is in particular F. The substituted radicalsalso include perfluorinated radicals. In the case of monosubstitution,the halogen substituent may be in any desired position, preferably inthe ω-position. Y¹¹ is particularly preferably F, Cl, CF₃, OCHF₂ orOCF₃, in particular F.

Z¹¹ is a single bond or is —CH₂—CH₂—, —CH═CH—, —CH═CF—, —CF═CH—,—CF═CF—, —C≡C—, —COO—, —OCO—, —CF₂O— or —OCF₂—. Z¹¹ is preferably asingle bond or is —COO— or —CF₂O—; Z¹¹ is particularly preferably—CF₂O—.

In the compounds of the formula I and of the formula IA,

is

is preferably a 1,4-cyclohexylene or 1,4-phenylene ring, while

is preferably a 1,4-phenylene ring which is unsubstituted ormonosubstituted or polysubstituted by fluorine.

a in the formula I can be 0 or 1. a is particularly preferably 1.

If f in the formula II is zero, the compounds of the formula II areterphenyls, which—depending on the meaning of b, c and d—may also carryone or more fluorine substituents. If, by contrast, f in the formula IIis 1, the compounds of the formula II are in the form of quaterphenyls,which may themselves—depending on the meaning of b, c, d and e—bemonosubstituted or polysubstituted by fluorine. It is also possible forthe liquid-crystalline media according to the invention simultaneouslyto comprise at least one terphenyl and one quaterphenyl of the formulaII. The media according to the invention preferably comprise eitherterphenyls of the formula II (f=0) or quaterphenyls of the formula II(f=1). If f=0, at least one or two of b, c and d is/are preferably 1 or2. If f=1, e for compounds of the formula II is preferably 0, i.e. thecorresponding phenyl ring is not substituted by F, while at least two ofb, c and d are preferably 1 or 2. In a preferred embodiment of theinvention, b, c, d and e are selected in such a way that b+c+d+e≧3; thesum of the lateral fluorine substituents of compounds of the formula IIis particularly preferably 3, 4, 5 or 6.

The compounds of the formulae I and II are prepared by methods known perse, as described in the literature (for example in the standard works,such as Houben-Weyl, Methoden der Organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants which are known per se, but arenot mentioned here in greater detail.

The compounds of the formula II can, for example, also be built up bySuzuki cross-coupling—which can also be carried out consecutively—ofcorresponding aromatic boronic acids or boronic acid esters withsuitably substituted phenyl compounds. Scheme 1 shows by way of examplesynthetic routes for the preparation of compounds of the formula II viaSuzuki cross-coupling. R²¹, R²², b, c, d and e here are as defined abovefor the formula II. M is Si, Ge or Sn. Further preparation processes aredescribed, inter alia, in German Patent Application 10211597.4 andEuropean Patent Application 03003811.1.

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance located in the cell) which contain media of thistype, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant broadening of the available parameter latitude. Theachievable combinations of clearing point, viscosity at low temperature,thermal and UV stability and dielectric anisotropy far exceed previousmaterials from the prior art.

In particular, the UV stability of the liquid-crystal media according tothe invention is significantly improved compared with the knownmaterials of the prior art, with the further desired or necessaryparameters generally not only not being impaired, but likewise beingsignificantly improved. Thus, in liquid-crystalline mixtures comprisingthe liquid-crystal medium according to the invention, a smaller drop inthe voltage holding ratio (VHR) [S. Matsumoto et al., Liquid Crystals 5,1320 (1989); K. Niwa et al., Proc. SID Conference, San Francisco, June1984, p. 304 (1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)]after UV irradiation is observed than in the case of conventionalmixtures. A comparable situation also applies to the change in thespecific resistance (SR) of the mixtures as a consequence of UVtreatment: liquid-crystal mixtures which comprise the liquid-crystallinemedium according to the invention have a significantly higher specificresistance after UV treatment and prove to be less radiation-sensitivethan mixtures without the medium according to the invention.

Liquid-crystal mixtures whose photostability may be reduced afteraddition of a compound of the formula I where Z¹¹=—(CH₂)₂—, —COO—,—OCO—, —CH═CH—, —CF═CH—, —CH═CF—, —CF═CF—, —C≡C—, —OCF₂— or —CF₂O— alsoexhibit significantly improved UV stability if, besides the compound (orcompounds) of the formula I (or IA), at least one compound of theformula II as defined above is employed in these mixtures.

The combination of the compounds of the formulae I and II seems toinfluence the various parameters of relevance for use in electro-opticaldisplays in their combination very much better than known materials fromthe prior art.

The mixtures according to the invention are preferably suitable asTN-TFT mixtures for notebook PC applications with 3.3 and 2.5 V drivers.They can also be used as TFT mixtures for projection applications, bothin transmissive and in reflective mode.

The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δε has hitherto only been achieved to aninadequate extent. The liquid-crystal mixtures according to theinvention, while retaining the nematic phase down to −20° C. andpreferably down to −30° C., particularly preferably down to −40° C.,enable a clearing point above 60° C., preferably above 65° C.,particularly preferably above 70° C., simultaneously usually dielectricanisotropy values Δε of ≧6, preferably ≧8, and a high value for thespecific resistance to be achieved, enabling excellent STN and MLCdisplays to be obtained. In particular, the mixtures are characterisedby low operating voltages. The TN thresholds are below 2.0 V, preferablybelow 1.5 V, particularly preferably <1.3 V.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 110°) to be achieved at ahigher threshold voltage or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having greater Δε and thus lowerthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975] are used, where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 3,022,818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables very high specificresistances to be achieved using the mixtures according to the inventionat the first minimum. Through a suitable choice of the individualcomponents and their proportions by weight, the person skilled in theart is able to set the birefringence necessary for a pre-specified layerthickness of the MLC display using simple routine methods.

The nematic phase range is preferably at least 90° C., in particular atleast 100° C. This range preferably extends at least from −20° to +80°C.

A short response time is desired in liquid-crystal displays. Thisapplies in particular to displays which are capable of videoreproduction. For displays of this type, response times (total:t_(on)+t_(off)) of at most 16 ms are required. The upper limit of theresponse time is determined by the image refresh frequency.

The formula I preferably covers compounds of the formula IA, inparticular the following compounds:

in which R¹¹ is as defined above for the formulae I and IA. R¹¹ ispreferably CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃, n-C₇H₁₅, CH₂═CH,CH₃CH═CH or 3-alkenyl (i.e. an alkenyl radical which has the C═C doublebond in the 3-position, such as, for example, in CH₂═CH—CH₂—CH₂—), inparticular C₂H₅, n-C₃H₇, n-C₄H₉ or n-C₅H₁₁.

Preference is given to media according to the invention which compriseat least one compound of the formulae IA1, IA5, IA9, IA17, IA40, IA41,IA42, IA47, IA52 and/or IA53, particularly preferably in each case atleast one compound of the formula IA47, particular preference beinggiven to the compounds in which R¹¹ is a straight-chain alkyl radicalhaving from 1 to 7 carbon atoms.

Further preferred compounds of the formula I are compounds of theformulae I1 to I8:

in which R¹¹ is as defined above for the general formulae I and IA. R¹¹is preferably CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃, n-C₇H₁₅,CH₂═CH, CH₃CH═CH or 3-alkenyl, in particular C₂H₅, n-C₃H₇, n-C₄H₉ orn-C₅H₁₁.

It is furthermore preferred for the liquid-crystalline medium accordingto the invention to comprise at least two compounds of the formulae Iand/or IA.

The formula II preferably covers the following compounds:

in which R²¹ and R²² are as defined above.

R²¹ and R²² in the compounds of the formulae II1 to II15 are preferably,independently of one another, F, Cl, CF₃ or straight-chain alkyl havingfrom 1 to 7 carbon atoms. R²¹ is, in particular, CH₃, C₂H₅, n-C₃H₇,n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃ or n-C₇H₁₅, very particularly preferably C₂H₅,n-C₃H₇, n-C₄H₉ or n-C₅H₁₁, while R²² is, in particular, F, Cl, CF₃, CH₃,C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃ or n-C₇H₁₅, very particularlypreferably F, C₂H₅, n-C₃H₇, n-C₄H₉ or n-C₅H₁₁. Of these preferredcompounds, the compounds of the formulae II2, II3, II9 and II10 whereR²¹ and R²², independently of one another, are C₁₋₇-alkyl and of theformula II15 where R²¹ is C₁₋₇-alkyl and R²² is F or C₁₋₇-alkyl areparticularly preferred.

It is furthermore preferred for the medium according to the inventionadditionally to comprise at least one compound of the formula III:

in which

L³¹ is H or F;

R³¹ is H, a halogenated or unsubstituted alkyl radical having from 1 to15 carbon atoms, where one or more CH₂ groups in these radicals may alsobe replaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way thatO atoms are not linked directly to one another;

R³² is H, F, Cl, a halogenated or unsubstituted alkyl radical havingfrom 1 to 15 carbon atoms, where one or more CH₂ groups in theseradicals may also be replaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO—in such a way that O atoms are not linked directly to one another; and

j is 0 or 1.

If R³¹ and/or R³² is an alkyl or alkoxy radical, this may bestraight-chain or branched. It is preferably straight-chain, has 1, 2,3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably methyl,ethyl, propyl, butyl, pentyl, hexyl, heptyl, methoxy, ethoxy, propoxy,butoxy, pentoxy, hexyloxy or heptyloxy, furthermore octyl, nonyl, decyl,undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, octyloxy, nonyloxy,decyloxy, undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),ethoxymethyl or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-,3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6-or 7-oxaoctyl, 2-, 3-, -4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-,6-, 7-, 8 8- or 9-oxadecyl.

If R³¹ and/or R³² is an alkyl radical in which one CH₂ group has beenreplaced by —CH═CH—, this may be straight-chain or branched. A radicalof this type is also known as an alkenyl radical. It is preferablystraight-chain and has from 2 to 10 carbon atoms. Accordingly, it isparticularly preferably vinyl, prop-1- or -2-enyl, but-1-, -2- or-3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl,hept-1-, -2-, -3-, 4-, -5- or -6-enyl, oct-1-, -2-, -3-, 4-, -5-, -6- or-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-,-2-, -3-, 4-, -5-, -6-, -7-, -8- or -9-enyl. If the alkenyl group canhave the E or Z configuration, the E configuration (trans configuration)is generally preferred.

If R³¹ and/or R³² is an alkyl radical in which one CH₂ group has beenreplaced by —O— and one has been replaced by —CO—, these are preferablyadjacent. These thus contain an acyloxy group —CO—O— or an oxycarbonylgroup —O—CO. These are preferably straight-chain and have from 2 to 6carbon atoms. Accordingly, they are in particular acetoxy, propionyloxy,butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl,2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R³¹ and/or R³² is an alkyl radical in which one CH₂ group has beenreplaced by unsubstituted or substituted —CH═CH— and an adjacent CH₂group has been replaced by CO, CO—O or O—CO, this may be straight-chainor branched. It is preferably straight-chain and has from 4 to 12 carbonatoms. Accordingly, it is in particular acryloyloxymethyl,2-acryloyloxyethyl, 3-acryloyloxypropyl, 4acryloyloxybutyl,5acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl,8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl,methacryoyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl,4-methacryloyloxybutyl, 5methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

If R³¹ and/or R³² is an alkyl, alkoxy or alkenyl radical which is atleast monosubstituted by halogen, this radical is preferablystraight-chain, and halogen is preferably F or Cl. In the case ofpolysubstitution, halogen is preferably F. The resultant radicals alsoinclude perfluorinated radicals. In the case of monosubstitution, thefluorine or chlorine substituent may be in any desired position, but ispreferably in the ω-position. Particular preference is given to —CF₃,—CHF₂, —CH₂F, —CH₂CF₃, —CHFCHF₂, —CF₂CF₃, —OCF₃, —OCHF₂, —OCH₂F and—CF═CF₂.

Compounds containing branched wing groups R³¹ and/or R³² mayoccasionally be of importance owing to better solubility in the usualliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

Branched groups of this type preferably contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

If R³¹ and/or R³² is an alkyl radical in which two or more CH₂ groupshave been replaced by —O— and/or —CO—O—, this may be straight-chain orbranched. It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)pentyl.

R³¹ is preferably H or methyl, ethyl, n-propyl or n-butyl, in particularH. R³² is preferably F, Cl or straight-chain alkyl having 1, 2, 3, 4, 5,6 or 7 carbon atoms, in particular F or n-propyl or n-pentyl.

Particularly preferred compounds of the formula III are

where n and m, independently of one another, are 0, 1, 2, 3, 4, 5, 6 or7. Very particular preference is given to the compounds of the formulaeIII1a and III1b where m=3, 4 or 5, compounds of the formula III2Aa wherem=2, 3, 4 or 5, and the compound of the formula III3a:

It is furthermore preferred for the liquid-crystalline media accordingto the invention to comprise one or more compounds of the formulae IVand/or V:

in which

R⁴¹, R⁴² and R⁵¹, independently of one another, are alkyl having from 1to 12 carbon atoms.

R⁴¹, R⁴² and R⁵¹ are preferably, independently of one another, linearalkyl chains each having from 1 to 7 carbon atoms, i.e. methyl, ethyl,n-propyl, n-butyl, n-pentyl, n-hexyl or n-heptyl, particularlypreferably n-propyl, n-butyl or n-pentyl.

In addition, it is preferred for the liquid-crystalline media accordingto the invention to comprise at least one compound of the formulae VIand/or VII and/or VIII:

in which

R⁶¹, R⁷¹ and R⁸¹, independently of one another, are alkyl having from 1to 12 carbon atoms.

R⁶¹, R⁷¹ and R⁸¹ are preferably, independently of one another,unbranched alkyl each having from 1 to 7 carbon atoms, i.e. methyl,ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl or n-heptyl, particularlypreferably ethyl, n-propyl, n-butyl or n-pentyl.

The compounds of the formulae VI, VII and VIII also each form apreferred group of compounds of the formula I (where, inter alia,Z¹¹=single bond for the formulae VII and VIII or Z¹¹=—CH₂CH₂— for theformula VI). In certain embodiments of the invention, the compounds ofthe formula I are selected from the group formed by the compounds of theformulae VI, VII and VIII. In other, particularly preferred embodiments,the compounds of the formulae VI, VII and/or VIII are present in theliquid-crystalline medium according to the invention besides at leastone compound of the formula I which is not also represented by one ofthe formulae VI, VII and VIII; this applies, in particular, if thecompound(s) of the formula I is (are) one or more compounds of theformula IA.

The joint proportion of the compounds of the formulae I and II in theliquid-crystal mixture as a whole is preferably from 5 to 85% by weight,in particular from 10 to 75% by weight, particularly preferably from 15to 65% by weight.

The proportion of the compounds of the formula II in the liquid-crystalmixture as a whole is preferably from 0.1 to 10% by weight, inparticular from 0.25 to 5% by weight and particularly preferably from0.5 to 2% by weight.

In preferred embodiments, the liquid-crystalline media according to theinvention, besides the compounds of the formula I, in particular of theformula IA, and of the formula II and any further constituents, forexample compounds of the formulae II, IV, V, VI, VII and/or VIII,furthermore comprise compounds of the formulae IX and/or X:

in which

R⁹¹ and R¹⁰¹ are each, independently of one another, H, a halogenated orunsubstituted alkyl radical having from 1 to 15 carbon atoms, where, inaddition, one or more CH₂ groups in these radicals may each be replaced,independently of one another, by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO—in such a way that O atoms are not linked directly to one another;

X⁹¹ and X¹⁰¹ are each, independently of one another, F, Cl, CN, SF₅,SCN, NCS, a halogenated alkyl radical, a halogenated alkenyl radical, ahalogenated alkoxy radical or a halogenated alkenyloxy radical, eachhaving up to 6 carbon atoms;

Z¹⁰¹ and Z¹⁰² are each, independently of one another, —CF₂O—, —OCF₂— ora single bond, where Z¹⁰¹≠Z¹⁰²;

L⁵, L⁶, L⁷, L⁸, L⁹ and L¹⁰ are each, independently of one another, H orF; and

are each, independently of one another,

At the same time, the compounds of the formula IX form a preferred groupof compounds of the formula I (where, inter alia, Z¹¹=—COO—). In certainpreferred embodiments of the invention, the compounds of the formula Iare selected from the group formed by the compounds of the formula IX.In other, particularly preferred embodiments, one or more compounds ofthe formula IX are present in the liquid-crystalline medium according tothe invention besides at least one compound of the formula I which isnot also represented by the formula IX; this applies, in particular, ifthe compound(s) of the formula I is (are) one or more compounds of theformula IA.

Preferred compounds of the formula IX are the following compounds:

In these formulae, R⁹¹ is as defined above for the formula IX. R⁹¹ ispreferably H, CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃, CH₂═CH,CH₃CH═CH or 3-alkenyl.

The compounds of the formula X preferably have the structure of theformula XA:

In this formula, R¹⁰¹ and X¹⁰¹ are as defined above for the formula X,and L¹¹, L¹², L¹³, L¹⁴ and L¹⁵, independently of one another, are H orF. Of these compounds of the formula XA, those of the following formulaeXA1 to XA24 are particularly preferred:

In these formulae, R¹⁰¹ is as defined above for the formulae X and XAand is preferably straight-chain alkyl having from 1 to 7 carbon atoms,in particular CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, n-C₆H₁₃ or n-C₇H₁₅,furthermore 1E- or 3-alkenyl, in particular CH₂═CH, CH₃CH═CH,CH₂═CHCH₂CH₂ or CH₃CH═CH—CH₂CH₂.

It is furthermore preferred for the medium according to the inventionadditionally to comprise one or more compounds selected from the groupconsisting of compounds of the general formulae XI to XVII:

in which the individual radicals have the following meanings:

R⁰ is H, n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9carbon atoms;

X⁰ is F, Cl, halogenated alkyl, alkenyl, alkenyloxy or alkoxy having upto 6 carbon atoms;

Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —CF₂O—, —OCF₂—, —OCH₂— or—CH₂O—;

Y¹, Y², Y³ and Y⁴ are each, independently of one another, H or F; and

r is 0 or 1.

(Some compounds of the formulae XI, XII, XIV, XV and XVII may—given acorresponding choice of X⁰, Z⁰, Y¹, Y², Y³, Y⁴ and r—in each case form agroup of compounds of the formula I (and/or of the formula IA). Incertain preferred embodiments of the invention, the compounds of theformula I (or, with regard to certain compounds of the formulae XII, XVand XVII, also of the formula IA) are selected from the groups formed bythe compounds of the formulae XI, XII, XIV, XV and XVII; this applies,in particular, to compounds of the formulae XIl, XV and XVII where Y¹═F,Y²═H or F and, where present, Z⁰=—CF₂O—. In other, particularlypreferred embodiments, one or more compounds of the formulae XI, XII,XIV, XV and/or XVII are present in the liquid-crystalline mediaaccording to the invention besides at least one compound of the formulaI or in particular of the formula IA which is not represented by theformulae XI, XII, XIV, XV and/or XVII.)

Preferred compounds of the formula XII are:

in which R⁰ is as defined above.

Preferred compounds of the formula XV are:

in which R⁰ is as defined above.

The medium preferably additionally comprises one or more compoundsselected from the group consisting of the general formulae XVIII toXXIV:

in which R⁰, X⁰, Y¹ and Y² are each, independently of one another, asdefined above for one of the formulae XI to XVII. Y³ is H or F. X⁰ ispreferably F, Cl, CF₃, OCF₃ or OCHF₂. R⁰ is preferably alkyl, oxaalkyl,fluoroalkyl or alkenyl, each having up to 6 carbon atoms.

The medium preferably additionally comprises one or more ester compoundsof the formulae Ea to Ec (if they are not already present as compound(s)of the formula I or IA)

in which R⁰ is as defined above for one of the formulae XI to XVII.

is preferably

The medium additionally comprises one, two, three or more, preferablytwo or three, compounds of the formula O1

in which “alkyl” and “alkyl*” are each, independently of one another, astraight-chain or branched alkyl radical having 1-9 carbon atoms.

The proportion of the compounds of the formula O1 in the mixturesaccording to the invention is preferably 5-10% by weight.

The medium preferably comprises one, two or three, furthermore four,homologues of the compounds selected from the group consisting of H1 toH12 (where n=1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12):

The medium comprises further compounds, preferably selected from thefollowing group consisting of compounds of the formulae RI to RIX:

in which

R⁰ is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each havingup to 9 carbon atoms;

Y¹, Y³ and Y⁵, independently of one another, are H or F;

“alkyl” and “alkyl*” are each, independently of one another, astraight-chain or branched alkyl radical having 1-9 carbon atoms;

“alkenyl” is a straight-chain or branched alkenyl radical having up to 9carbon atoms.

The medium preferably comprises one or more compounds of the followingformulae:

in which n and m, independently of one another, are 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11 or 12. n and m are preferably, independently of oneanother, 1, 2, 3, 4, 5 or 6. In the case of the formula RIXa, n is inparticular 2.

In a further embodiment, it is preferred for the medium according to theinvention to comprise one or more compounds having fused rings, of theformulae AN1 to AN11:

in which R⁰ is as defined above.

It has been found that the media according to the invention comprisingcompounds of the formulae I and II mixed with conventionalliquid-crystal materials, but in particular with one or more compoundsof the formulae III, IV, V, VI, VII and/or VIII and of the formulae IX,X, XI, XII, XIII, XIV, XV, XVI and/or XVII, results in very goodstability, a significant lowering of the threshold voltage and in highvalues for the VHR values (100° C.), with broad nematic phases with lowsmectic-nematic transition temperatures being observed at the same time,improving the shelf life. The compounds of the formulae I to XVII arestable and readily miscible with one another and with otherliquid-crystalline materials.

The term “alkyl” or “alkyl*”—unless defined otherwise elsewhere in thisdescription or in the claims—covers straight-chain and branched alkylgroups having 1-12, preferably 1-7 carbon atoms, in particular thestraight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl andheptyl. Groups having 1-5 carbon atoms are generally preferred.

The term “alkenyl”—unless defined otherwise elsewhere in thisdescription or in the claims—covers straight-chain and branched alkenylgroups having 2-12, preferably 2-7 carbon atoms, in particular thestraight-chain groups. Preferred alkenyl groups are C₂-C₇-1E-alkenyl,C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, inparticular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.Examples of particularly preferred alkenyl groups are vinyl,1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl,3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. Preferably, n=1 and m is from1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio of the elastic constantsk₃₃ (bend) and k₁₁ (splay) compared with alkyl or alkoxy radicals.4-alkenyl radicals, 3-alkenyl radicals and the like generally give lowerthreshold voltages and smaller values of k₃₃/k₁₁ compared with alkyl andalkoxy radicals.

A —CH₂CH₂— group generally results in higher values of k₃₃/k₁₁ comparedwith a single covalent bond. Higher values of k₃₃/k₁₁ facilitate, forexample, flatter transmission characteristic lines in TN cells with a90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

A preferred aspect of the invention relates to liquid-crystalline mediaaccording to the invention which are present in a liquid-crystal mixturewhich is distinguished by a high value of the optical anisotropy Δn,preferably of >0.16, in particular of >0.20. So-called “high Δn”liquid-crystal mixtures of this type are used, in particular, in OCB(optical compensated birefringence), PDLC (polymer dispersed liquidcrystal), TN and STN cells, in particular for achieving small layerthicknesses in TN and STN cells for shortening the response times. Inthese “high Δn” liquid-crystal mixtures, the liquid-crystalline mediumaccording to the invention comprises compounds of the formula I whichcontribute to increasing the optical anisotropy Δn besides at least onecompound of the formula II. These compounds of the formula I arepreferably one or more of the following compounds:

R¹¹ here is as defined above for the formulae I and IA. These mediaaccording to the invention may furthermore comprise further compounds,in particular of the formulae III, IV, V, VI, VII, VIII, IX, X, XI, XII,XIII, XIV, XV, XVI and/or XVII, if the resultant mixture has asufficiently high value of the optical anisotropy Δn for the preferreduse as “high Δn” mixture.

The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI+VII+VIII (optionally +IX+X+XI+XII+XIII+XIV+XV+XVI+XVII)depends substantially on the desired properties, in particular on thedesired use, on the choice of these components and the choice of anyother components that may be present. Suitable mixing ratios can easilybe determined from case to case.

The total amount of compounds of the formulae I to XVII in the mixturesaccording to the invention is not crucial. The mixtures may thereforecomprise one or more further components for the purposes of optimisationof various properties.

The individual compounds of the above-mentioned formulae and subformulaewhich can be used in the media according to the invention are eitherknown or can be prepared analogously to the known compounds.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term conventional construction is broadly drawn here and also coversall derivatives and modifications of the MLC display, in particularincluding matrix display elements based on poly-Si TFT or MIM.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

Display devices can be filled with the liquid-crystal mixture comprisingthe liquid-crystalline medium according to the invention in aconventional manner, but in particular by the “one-drop filling method”.

The liquid-crystal mixtures according to the invention are prepared in amanner conventional per se. In general, the desired amount of thecomponents used in the lesser amount is dissolved in the componentsmaking up the principal constituent, advantageously at elevatedtemperature. It is also possible to mix solutions of the components inan organic solvent, for example in acetone, chloroform or methanol, andto remove the solvent again, for example by distillation, after thoroughmixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature. For example, 0-15%of pleochroic dyes or chiral dopants can be added.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m carbon atoms respectively;n and m are integers and are preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11 or 12. The coding in Table B is self-evident. In Table A, onlythe acronym for the parent structure is indicated. In individual cases,the acronym for the parent structure is followed, separated by a dash,by a code for the substituents R^(1*), R^(2*), L^(1*) and L^(2*).

Code for R¹*, R²*, L¹*, L²* R¹* R²* L¹* L²* nm C_(n)H_(2n+1)C_(m)H_(2m+1) H H nO•m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H nOmC_(n)H_(2n+1) OC_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN•FC_(n)H_(2n+1) CN F H nN•F•F C_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H HnCL C_(n)H_(2n+1) Cl H H nF•F C_(n)H_(2n+1) F F H nF•F•F C_(n)H_(2n+1) FF F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₃•F C_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m+1) H H nV-Vm C_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1)H H

Preferred mixture components are given in Tables A and B.

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CFU

FET-nCL

TABLE B

BCH-n•Fm

CFU-n-F

CBC-nm

CBC-nmF

ECCP-nm

CCZU-n-F

T-nFm

CGU-n-F

CCH-nm

CCH-nCF₃

PCH-nm

GGP-n-m

GGP-n-CL

PGP-n-m

CDU-n-F

CGG-n-F

CPZG-n-OT

CC-nV-Vm

CCP-Vn-m

CCP-nm

CCP-nOCF₃

CCP-nOCF₃•F

CCP-nF•F•F

BCH-nF•F

BCH-nF•F•F

FET-nCL

FET-nF

CCG-V-F

CCP-nV-m

CC-n-V

CCQU-n-F

CC-n-V1

CCQG-n-F

CQCU-n-F

DecU-n-F

CWCU-n-F

CWCG-n-F

CCOC-n-m

CPTU-n-F

GPTU-n-F

PQU-n-F

PUQU-n-F

PGU-n-F

CGZP-n-OT

CCGU-n-F

CCQG-n-F

CUQU-n-F

PUQU-n-F

CUQU-n-F

PYGP-n-m

YGGP-n-m

PGIGI-n-F

CGUQU-n-F

CPUQU-n-F

PGGU-n-F

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formula I, comprise at least one, two,three or four compounds from Table B.

TABLE C Table C shows possible dopants which are generally added to themixtures according to the invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

R/S-1011

R/S-3011

CN

R/S-2011

R/S-4011

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention are mentioned below.

Above and below, percentages are per cent by weight. All temperaturesare given in degrees Celsius. m.p. denotes melting point, cl.p. denotesclearing point. Furthermore, C=crystalline state, N=nematic phase,S=smectic phase and I=isotropic phase. S_(C) denotes a smectic C phase.The data between these symbols represent the transition temperatures.V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.0 times the value of V₁₀. Δn denotes the optical anisotropy(Δn=n_(e)−n_(o), where n_(e) is the refractive index of theextraordinary ray and n_(o) is the refractive index of the ordinary ray)(589 nm, 20° C.). Δε denotes the dielectric anisotropy (Δε=ε_(∥)−ε_(⊥),where ε_(∥) denotes the dielectric constant parallel to the longitudinalmolecular axes and ε_(⊥) denotes the dielectric constant perpendicularthereto) (1 kHz, 20° C.). The electro-optical data were measured in a TNcell at the 1st minimum (i.e. at a d·Δn value of 0.5 μm) at 20° C.,unless expressly stated otherwise. The optical data were measured at 20°C., unless expressly stated otherwise. The flow viscosity ν₂₀ (mm²/sec)was determined at 20° C. The rotational viscosity γ₁ [mPa·s] waslikewise determined at 20° C. SR denotes the specific resistance [Ω.cm].The voltage holding ratio VHR was determined under thermal load at 100°C. or with UV irradiation (wavelength>300 nm; irradiation intensity 765W/m²) for 2 hours. The physical parameters were determinedexperimentally as described in “Licristal, Physical Properties Of LiquidCrystals, Description of the Measurement Methods”, Ed. W. Becker, MerckKGaA, Darmstadt, revised edition, 1998.

The following examples are intended to explain the invention withoutrestricting it.

EXAMPLES Example 1

a) Without a compound of the formula II: mixture M1

CCP-2OCF3•F 14.00%  S→N [° C.]: <−30 CCP-2F•F•F 10.00%  Clearing point[° C.]: +85 CCP-3F•F•F 14.00%  n_(e) [589 nm; 20° C.]: 1.5468 CCP-5F•F•F6.00% n_(o) [589 nm; 20° C.]: 1.4707 CCZU-2-F 4.00% Δn [589 nm; 20° C.]:+0.0761 CCZU-3-F 17.00%  ε_(∥) [1 kHz; 20° C.]: 15.3 CCZU-5-F 4.00%ε_(⊥) [1 kHz; 20° C.]: 4.3 CGU-2-F 6.00% Δε [1 kHz; 20° C.]: +11.0CGZP-2-OT 8.00% VHR (100° C.): 94.6 CCH-5CF3 8.00% VHR (2 h UV irrad.):92.3 CCOC-4-3 3.00% CCOC-3-3 3.00% CCOC-3-5 3.00%

b) With a compound of the formula II:

Various amounts of a compound of the formula II were added to the abovemixture M1, and the VHR was determined after UV irradiation for 2 hours:

Mixture VHR (2 h UV irradiation) [%] M1 without a compound of the 92.3formula II M1 + 0.60% by wt. of PYGP-4-3 94.7 M1 + 0.62% by wt. ofYGGP-4-3 94.9 M1 + 0.69% by wt. of PYGP-4-4 95.0

Example 2

a) Without a compound of the formula II: mixture M2

GGP-3-CL 9.00% S→N [° C.]: <−20 GGP-5-CL 24.00%  Clearing point [° C.]:+100 PGIGI-3-F 4.00% n_(e) [589 nm; 20° C.]: 1.7162 BCH-3F•F 4.00% n_(o)[589 nm; 20° C.]: 1.5161 BCH-5F•F 5.00% Δn [589 nm; 20° C.]: +0.2001BCH-3F•F•F 9.00% ε_(∥)[1 kHz; 20° C.]: 16.9 BCH-5F•F•F   10.00%% ε_(⊥)[1kHz; 20° C.]: 4.6 CBC-33F 3.00% Δε [1 kHz; 20° C.]: +12.3 CBC-53F 4.00%k₁ [pN]: 14.5 CCG-V-F 6.00% k₃ [pN]: 15.3 PGU-2-F 6.00% k₃/k₁: 1.05PGU-3-F 7.00% γ₁ [mPa · s; 20° C.]: 327 FET-2CL 5.00% V₁₀ [V]: 1.15FET-3CL 4.00%

b) With a compound of the formula II:

Various amounts of a compound of the formula II were added to the abovemixture M2, and the VHR was determined after UV irradiation for 2 hours:

Mixture VHR (2 h UV irradiation) [%] M2 without a compound of the 96.0formula II M2 + 0.53% by wt. of PYGP-4-3 97.1 M2 + 0.99% by wt. ofPYGP-4-3 97.5 M2 + 1.51% by wt. of PYGP-4-3 97.6 M2 + 1.98% by wt. ofPYGP-4-3 97.3

Example 3

a) Without a compound of the formula II: mixture M3

CC-5-V 10.00% Clearing point [° C.]: +55 PCH-53 30.00% n_(e) [589 nm;20° C.]: 1.5545 CCP-2F.F.F 15.00% n_(o) [589 nm; 20° C.]: 1.4837CCP-3F.F.F 15.00% Δn [589 nm; 20° C.]: +0.0708 CCP-5F.F.F 10.00% ε_(∥)[1kHz; 20° C.]: 6.8 CCP-31 10.00% ε_(⊥)[1 kHz; 20° C.]: 3.0 CCG-V-F 10.00%Δε [1 kHz; 20° C.]: +3.8

b) With a compound of the formula II:

20% by weight of PUQU-3-F (=mixture M3A) and various amounts of acompound of the formula II were added to the above mixture M3, and theVHR was determined after UV irradiation for 2 hours:

Mixture VHR (2 h UV irradiation) [%] M3A without a compound of the 94.2formula II M3 + 0.5% by wt. of PYGP-4-3 96.4 M3 + 1.0% by wt. ofPYGP-4-3 96.4

Furthermore, the specific resistance SR of mixture M3A without and withaddition of a compound of the formula II was determined in a glass cellafter UV irradiation:

SR [Ω · cm] SR [Ω · cm] SR [Ω · cm] Mixture after 0 h after 10 h after50 h M3A without a compound 3.5 · 10¹⁴   2 · 10¹³ 4 · 10¹⁰ of theformula II M3A + 0.5% by wt. of 1.0 · 10¹⁴ 1.5 · 10¹⁴ 6 · 10¹² PYGP-4-3M3A + 1.0% by wt. of 1.0 · 10¹⁴ 1.7 · 10¹⁴ 3 · 10¹³ PYGP-4-3 M3A + 1.0%by wt. of 1.3 · 10¹⁴ 1.0 · 10¹⁴ 2.5 · 10¹²   PGP-2-2 M3A + 1.0% by wt.of 2.5 · 10¹⁴   7 · 10¹³ 1.5 · 10¹²   GGP-5-3

Example 4

a) Without a compound of the formula II: mixture M4

CCH-35 3.00% CC-3-V1 4.00% Clearing point [° C.]: +84 CCP-1F•F•F 10.00% n_(e) [589 nm; 20° C.]: 1.5672 CCP-2F•F•F 8.00% n_(o) [589 nm; 20° C.]:1.4745 CCP-3F•F•F 9.00% Δn [589 nm; 20° C.]: +0.0927 CCP-2OCF3•F 12.00% ε_(∥)[1 kHz; 20° C.]: 15.4 CCP-2OCF3 8.00% ε_(⊥)[1 kHz; 20° C.]: 4.0CCP-3OCF3 8.00% Δε [1 kHz; 20° C.]: +11.5 CCP-4OCF3 5.00% γ₁ [mPa · s;20° C.]: 132 CCP-5OCF3 7.00% PUQU-2-F 6.00% PUQU-3-F 9.50% CGU-2-F 3.50%CCGU-3-F 5.00% CBC-33 2.00%

b) With a compound of the formula II:

Various amounts of a compound of the formula II were added to the abovemixture M4, and the VHR was determined after UV irradiation for 2 hours:

Mixture VHR (2 h UV irradiation) [%] M4 without a compound of the 95.8formula II M4 + 0.5% by wt. of PYGP-4-3 96.6 M4 + 1.0% by wt. ofPYGP-4-3 97.2

Furthermore, the specific resistance SR of mixture M4 without and withaddition of a compound of the formula II was determined in a glass cellafter UV irradiation:

SR [Ω · cm] SR [Ω · cm] SR [Ω · cm] Mixture after 0 h after 10 h after50 h M4 without a compound 1.0 · 10¹⁴ 2.2 · 10¹³ 3 · 10¹⁰ of the formulaII M4 + 0.5% by wt. of 1.3 · 10¹⁴ 1.6 · 10¹⁴ 2 · 10¹³ PYGP-4-3 M4 + 1.0%by wt. of 2.0 · 10¹⁴ 2.3 · 10¹⁴ 1.2 · 10¹⁴   PYGP-4-3

1. A liquid-crystalline medium comprising at least one compound offormula I

 and at least one compound of formula II

 in which L¹, L², L³ and L⁴ are each, independently of one another, H orF; R¹¹ is H, a halogenated or unsubstituted alkyl radical having from 1to 15 carbon atoms, where, in addition, one or more CH₂ groups in theseradicals may each be replaced, independently of one another, by —C≡C—,—CH═CH—, —O—, —CO—O—or —O—CO— in such a way that O atoms are not linkeddirectly to one another; R²¹ and R²² are each, independently of oneanother, H, Cl, F, CN, SF₅, SCN, NCS, a halogenated or unsubstitutedalkyl radical having from 1 to 15 carbon atoms, where, in addition, oneor more CH₂ groups in these radicals may each be replaced, independentlyof one another, by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a waythat O atoms are not linked directly to one another; Y¹¹ is F, Cl, CN,SF₅, SCN, NCS, a halogenated alkyl radical, a halogenated alkenylradical, a halogenated alkoxy radical or a halogenated alkenyloxyradical, each having up to 6 carbon atoms; Z¹¹ is a single bond, a, is1; b, c, d and e are each, independently of one another, 0, 1 or 2; f is1;


2. The liquid-crystalline medium according to claim 1, wherein theproportion of the compounds of the formula II in the mixture as a wholeis from 0.1 to 10% by weight.
 3. An electro-optical liquid-crystaldisplay containing a liquid-crystalline medium according to claim
 1. 4.The liquid-crystalline medium according to claim 1 wherein theproportion of the compounds of formula II in the mixture as a whole is0.25 to 5% by weight.
 5. The liquid-crystalline medium according toclaim 1 wherein the proportion of the compounds of formula II in themixture as a whole is 0.5 to 2% by weight.
 6. The liquid crystallinemedium according to claim 1, wherein the compound of formula I is


7. The liquid crystalline medium according to claim 1, wherein thecompound of formula I is


8. The liquid crystalline medium according to claim 1, wherein thecompound of formula I is


9. The liquid crystalline medium according to claim 1, wherein thecompound of formula I is


10. The liquid crystalline medium according to claim 1, wherein thecompound of formula II is